1. Field of the Invention
This invention relates to a vibration detector used for such purposes as to detect the vibration of an internal-combustion engine and knocking of the same.
2. Description of the Prior Art
FIGS. 1 to 3 show a conventional vibration detector A. In these drawins, reference number 1 is a bushing made from a ferrous metallic material including a bolt threading hole 13 at its center portion. The bushing comprises a bearing surface 1a for connecting the detector A to an internal-combustion engine B, a block surfare 1b for attaching a vibration detecting part, a tube-like portion 1c which is a shaft for mounting the vibration detecting part, and a threaded portion 1d at the top portion of the tube-like portion 1c. Reference number 2 designates a retaining shield connected to the outer periphery of the bushing 1 by adhesion and defining a part receiving space C between the bushing 1 and itself. Reference number 3 is a connector provided integrally with the retaining shield 2 which includes an output terminal 11 for emitting detecting signals. Reference number 4 is a plate located on the block surface 1b of the bushing 1 and 5 is a piezoelectric element for converting vibrations into electric signals, the reference electrode of which is connected to the plate 4. Reference number 6 denotes a terminal in contact with an output electrode of the piezoelectric element 5 and adapted to output detecting signals, and 7 designates an isolating sheet made from a film such as PET (polyethylene terephthalate) or PPS (polyphenylene sulfide) which is located in contact with the side of the terminal 6 with which the piezoelectric element 5 has no contact. Reference number 8 is a weight which applies vibrational energy to the piezoelectric element 5; 9 a nut screwed to the tube-like portion 1c for the purpose of fixing the plate 4, the piezoelectric element 5, the terminal 6, the isolating sheet 7 and the weight 8 to the tube-like portion 1c; 10 an epoxy filler for sealing the receiving portion between the bushing 1 and retaining shield 2; and 12 an isolating tube formed from an adhesive tape or a heat shrinkable tube provided between the bush and the piezoelectric element 5, the terminal 6 and the isolating sheet 7. The detecting signal supplied by the piezoelectric element 5 is emitted by the terminal 6. Because the terminal 6 and the output terminal 11 are connected to each other, the detecting signal is output to the outside of the detector A through the connector 3.
The vibration detecting principle of the above-mentioned vibration detector A will now be described. As shown in FIG. 3, the vibration detector A is secured with its bearing surface 1a in contact with the internal-combustion engine B by means of a stud bolt D inserted through the hole 13 of the bushing 1 and a nut E which engages with the bolt D. The vibrations produced in accordance with the condition in which the internal-combustion engine B is driven are transmitted to the detector A from the bearilng surface 1a. Because the weight 8 overlaps the piezoelectric element 5, the inertial force of the weight 8 is communicated to the piezoelectric element 5 is accordance with the level of vibration. When the piezoelectric element 5 receives the inertial force, it generates a detecting signal representing that force and outputs the detecting signal to the electrode on the side of the terminal 6 on the basis of the reference electrode on the plate 4 side. Since the plate 4 is made from a metallic material, the reference electrode of the piezoelectric element 5 is equipotential with the bushing 1. On the other hand, since the nut 9 and the weight 8 are also metallic, the weight 8 is equipotential with respect to the reference electrode of the piezoelectric element 5. For this reason, the terminal 6 and the weight 8 are isolated by virtue of the presence of the isolating sheet 7, so the detecting signal supplied by the piezoelectric element 5 is emitted at the terminal 6 to be output via the output terminal 11. Because the isolating tube 12 is fitted on the tube-like portion 1c of the bush 1, the terminal is not short-circuited to the tube-like portion 1c of the bush 1 even if the terminal is eccentrically mounted on the tube-like portion. In this way, the isolating tube 12 can be seen as being important to emit the detecting signal securely. The interior which is surrounded by the block surface 1b, the plate 4, the piezoelectric element 5, the terminal 6, the isolating sheet 7, the weight 8 and the nut 9 on the periphery of the the tube-like portion 1c which is provided with the isolating tube 12 is not filled with the filler 10.
As clearly shown in FIG. 2, the diameter .phi..sub.2 of the portion of the tube-like portion 1c of the bush 1 apart from the threaded portion 1d is arranged to be equal to or larger than the major diameter of the threaded portion 1d. This arrangement is made for the following reason. That is, the portion of the tube-like portion 1c apart from the threaded portion 1d is manufactured in the same way over the full length thereof for manufacturing simplicity, and as it is necessary to perform a basic process on the tube-like portion 1c to form the threaded portion 1d, the diameter of the portion of the tube-like portion 1c apart from the threaded portion 1d is made equal to or larger than that of the threaded portion 1d.
Because the conventional vibration detector is formed in this way, in a case where the portion of the tube-like portion 1c of the bush 1 apart from the threaded portion 1d has to be lengthened, the overall weight of the bushing 1 is increased. It is then impossible to reduce the size and weight of either the bush 1 or the vibration detector as a whole.
Further, as shown in FIG. 3, in a case where the vibration detector A is attached to the internal-combustion engine B by means of the bolt D and nut E, the tube-like portion 1c of the bushing 1 is subjected to the compression force of the nut E and the tube-like portion 1c is therefore designed on the basis of the magnitude of the compression stress produced in the bush 1. As the strength of the tube-like portion 1c depends on the portion having the smallest diameter, the stength of the tube-like portion 1c against compression depends on the minimum diameter of the threaded portion 1d. Accordingly, even if the diameter of the portion of the tube-like portion 1c apart from the threaded portion 1d is enlarged, this only leads to an increase in the weight of the bushing 1 and the strength of the tube-like portion 1c against compression is not necessarily raised.